1. Field of the Invention
The present invention relates to a diffractive optical power region blended in a lens by decreasing the diffractive efficiency of the diffractive optical power region near the peripheral edge thereof. The present invention also relates to a diffractive optical power region that provides a progression of optical power. The present invention further relates to both static and dynamic multifocal lenses which may use continuous or discontinuous diffractive structures.
2. Description of the Related Art
A diffractive optical power region is a region of a lens or optic that generates optical power by diffracting light. A static diffractive optical power region comprises individual surface relief diffractive structures that are typically closed concentric curves. The surface relief diffractive structures are closely spaced (e.g., by a distance on the order of the wavelengths of visible light). The surface relief diffractive structures typically have the same heights. The surface relief diffractive structures may commonly be referred to as Fresnel zones.
In general, for a given thickness of a lens or optic, diffractive optical power regions are capable of generating greater optical power than their refractive counterparts. Despite this advantage, diffractive optical power regions have several disadvantages.
One of the main disadvantages of using diffractive optical power regions is that they exhibit large amounts of chromatic aberration compared to their refractive counterparts. Chromatic aberration refers to the change in optical power that occurs as the optical wavelength is varied. Chromatic aberration in a diffractive optical power region having a constant optical power increases as the diffractive structures approach the periphery of the lens. Thus, the periphery of the diffractive optical power region exhibits the highest degree of chromatic aberration.
As a result of such chromatic aberration, a lens having a diffractive optical power region provides serious vision compromises. A vision compromise can be seen, in one approach, when the diffractive region is used to create a bifocal lens. In this approach, a diffractive optical power region may be placed in optical communication with the bottom half of an ophthalmic lens. The lens has a far distance vision correction and the diffractive optical power region provides additional optical power for near distance correction. Thus, the periphery of the diffractive optical power region forms the boundary between far distance correction and near distance correction. Since this boundary has the highest degree of chromatic aberration, a user looking across the boundary will experience the highest degree of the compromised vision.
Another disadvantage of a diffractive optical power region is that it is generally considered to be cosmetically unattractive. In the bifocal approach described above, the lens would have a sharp delineation at the boundary between the diffractive optical power region and the ophthalmic lens, similar to the line in conventional bifocals, which can be observed on a wearer. A wearer typically finds this undesirable. Since the ophthalmic industry trends towards lineless multifocal lenses (e.g., progressive addition lenses), diffractive optical power regions are less cosmetically desirable.
Thus, there is a need in the art for a diffractive optical power region that resolves the aforementioned vision and cosmetic compromises. Accordingly, there is now provided with this invention an improved lens for effectively overcoming the aforementioned difficulties and longstanding problems inherent in the art.